Programmable logic controllers (PLCs) are programmable microprocessor-based devices that are used in discrete manufacturing to control assembly lines and machinery on a shop floor as well as many other types of mechanical, electrical, and electronic equipment in a plant. Using conventional reduced instruction set computer (RISC) based chip sets and programmed in an IEC 61131 programming language, PLCs are designed for realtime use in rugged, industrial environments. Connected to sensors and actuators, PLCs are categorized by a number and type of I/O ports they provide and by their I/O scan rate. PLCs were first used to replace hardwired networks of relays and timers in automobile assembly lines, which were partially automated at that time. The PLCs' programmability enables changes to be effected considerably faster than previous processes.